When using straight teeth with a belt-drive transmission device, there exist problems of generating a lot of noise and vibration. As a way to solve these problems, a belt-drive transmission device that applies helical teeth, for example, is known (KOKAI No. 4-330853 (refer to claims and FIG. 3B)). However, for a power transmission device, such as a reduction device that is driven under a heavy load or at a high-speed rotation (e.g. 1500 rpm to 4500 rpm), the noise and the vibration are not always effectively reduced when using helical teeth.